This symbol is not used anywhere, so remove it. For spear600, remove
it from the board file, since the symbol is not defined for spear600
either.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>